The two traditional broad-band fan noise generation mechanisms are “noise due to incident turbulence” and “noise due to vortex shedding.” These might be called “aerodynamicists' mechanisms,” since they are both rooted in fluctuating lift concepts derived from two-dimensional incompressible unsteady aerofoil theory. The paper looks critically at these concepts and evaluates the practical significance of the mechanisms in current and future fan designs. Thus, it discusses their relationship to the newer “acousticians' mechanisms,” which are more intimately concerned with the acoustical properties of a compressible flow, and at the same time assesses the importance of all broad-band mechanisms compared with discrete tone generators.